The proposed research program involves applications of magnetic resonance spectroscopy to investigate interactions between a lipolytic enzyme and aggregated phospholipids and between metabolic enzymes and their substrate molecules. The objective of the research is to determine structures of the complexes formed between enzyme and aggregated phospholipid-substrates and between enzymes and substrates in order to elucidate the mechanism of the respective catalytic reactions and factors which control or regulate enzyme activities. The lipolytic enzyme under investigation is phospholipase A2 from the digestive secretion of mammalian pancreas. The enzyme catalyzes hydrolysis of fatty acid esters of acylphosphoglycerides which are ubiquitous constituents of biological membranes. In addition to the role of phospholipases in digestion, tissue phospholipases are involved in maintenance of cellular functions through turnover and renewal of phospholipids. The enzyme is activated upon binding to an appropriate lipid-water interface and hydrolyzes water-insoluble substrates; however, the detailed mechanisms for these two phenomena are not understood. Structural studies of the complexes of phospholipase with aggregated lipids are tractable by magnetic resonance methods since the enzyme is small, stable, highly solubule in water, and forms complexes with micelles of short chain phospholipids. Adenylosuccinate synthetase catalyzes the first reaction in the biosynthetic pathway from IMP to AMP. Studies will focus on the organization of substrates at the active site and the role of substrate-induced conformational changes in coupling hydrolysis of GTP to the synthesis of adenylosuccinate from IMP and L-aspartate.